There is an increasing demand for high-strength hot-rolled steel sheet (with a tensile strength higher than 780 MPa) for automobiles from the standpoint of weight reduction (which leads to energy saving and good fuel economy) and improved safety in case of collision. The high-strength hot-rolled steel sheet for such uses is required to have good drawability as well as hole expandability. Thus there have been proposed several techniques to meet these requirements.
Among known high-strength hot-rolled steel sheets is the one which has a composite structure composed of residual austenite and martensite. For example, Patent Document 1 discloses a method of improving hole expandability of steel sheet of composite structure composed of ferrite, bainite, residual-austenite, and martensite by extremely reducing the P content, controlling the maximum size of microstructure and inclusions, and controlling the hardness of microstructure.
Patent Document 2 discloses a high-strength steel sheet of ferrite-bainite structure (with ferrite dominating) which contains an adequately controlled amount of unfixed carbon (which remains unreacted with Ti and Nb in steel) and unprecipitated carbon (which precipitates in grain boundaries at the time of ageing to increase strength). Patent Document 3 discloses a method of improving hole expandability by turning a high-strength hot-rolled steel sheet into one which has microstructure composed of ferrite (as a major component) and bainitic ferrite and polygonal ferrite. The disclosed method involves the condition and technique of cooling the hot-rolled sheet in the coiling step which are necessary to form the above-mentioned microstructure.
Patent Document 4 also discloses a method of improving hole expandability by turning a high-strength hot-rolled steel sheet into the one which has microstructure composed of bainitic ferrite and polygonal ferrite. The disclosed method involves the condition and technique of cooling the hot-rolled sheet in the coiling step which are necessary to form the above-mentioned microstructure.
Unfortunately, the techniques proposes so far are not able to improve hole expandability as desired.
Patent Document 1:
    Published Japanese Translation of PCT No. 2004-536965Patent Document 2:    Japanese Patent Laid-open No. 2003-342684Patent Document 3:    Japanese Patent Laid-open No. 2004-250749Patent Document 4:    Japanese Patent Laid-open No. 2004-225109